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| United States Patent |
5,151,703
|
|
Roos
|
September 29, 1992
|
Method and apparatus for improved automatic fequency control
Abstract
The frequency of a variable frequency oscillator in a pulsed radar is
adjusted using samples of an IF signal. An initial frequency adjustment is
made using a sample taken during RF transmission. This initial adjustment
contains an inaccuracy introduced by the magnetron's activity. An
additional adjustment is made using temperature dependent values stored in
a memory. Yet another adjustment is made during the radar's receive time
using signals reflected by targets.
| Inventors:
|
Roos; Mark G. (Shawnee, KS)
|
| Assignee:
|
Allied-Signal Inc. (Morris Township, Morris County, NJ)
|
| Appl. No.:
|
804280 |
| Filed:
|
December 6, 1991 |
| Current U.S. Class: |
342/199; 342/174 |
| Intern'l Class: |
G01S 007/03 |
| Field of Search: |
342/174,199
|
References Cited
U.S. Patent Documents
| 3611380 | Oct., 1971 | Carlsson | 342/199.
|
| 3938148 | Feb., 1976 | Hobson | 343/7.
|
| 3950751 | Apr., 1976 | Orr et al. | 343/18.
|
| 3983555 | Sep., 1976 | Morrison et al. | 342/199.
|
| 4071844 | Jan., 1978 | Hopwood et al. | 343/17.
|
| 4228434 | Oct., 1980 | Williamson et al. | 343/5.
|
| 4240076 | Dec., 1980 | Williamson | 343/7.
|
| 4484193 | Nov., 1984 | Bellew | 343/5.
|
| 4600924 | Jul., 1986 | Lobsinger et al. | 342/199.
|
| 4907000 | Mar., 1990 | Tabourier | 342/84.
|
Primary Examiner: Tubbesing; T. H.
Attorney, Agent or Firm: Malvone; Christopher N., Massung; Howard G., Walsh; Robert A.
Claims
I claim:
1. A method for providing automatic frequency control of a variable
frequency oscillator in a pulsed radar, comprising the steps of:
(a) sampling a first error voltage which indicates a frequency difference
between a desired intermediate frequency and an actual intermediate
frequency, said sampling occurring during a transmission of an RF pulse;
(b) modifying a control signal to the variable frequency oscillator so that
said actual intermediate frequency approaches said desired intermediate
frequency;
(c) sampling a second error voltage which indicates a second frequency
difference between a desired intermediate frequency and a second actual
intermediate frequency, said sampling occurring during a receive time of
the radar; and
(d) modifying said control signal to the variable frequency oscillator so
that said second actual intermediate frequency approaches said desired
intermediate frequency.
2. The method of claim 1, further comprising the steps of measuring an
ambient temperature and modifying said control signal based on said
ambient temperature.
3. The method of claim 2, wherein said step of modifying said control
signal based on said ambient temperature comprises reading a temperature
dependent value from a nonvolatile memory.
4. A method for providing automatic frequency control of a variable
frequency oscillator in a pulsed radar, comprising the steps of:
(a) sampling a first error voltage which indicates a frequency difference
between a desired intermediate frequency and an actual intermediate
frequency, said sampling occurring during a transmission of an RF pulse;
(b) modifying a control signal to the variable frequency oscillator so that
said actual intermediate frequency approaches said desired intermediate
frequency;
(c) measuring an ambient temperature; and
(d) modifying said control signal based on said ambient temperature.
5. The method of claim 4, wherein said step of modifying said control
signal based on said ambient temperature comprises reading a temperature
dependent value from a nonvolatile memory.
6. An apparatus for providing automatic frequency control of a variable
frequency oscillator in a pulsed radar, comprising:
(a) discriminator means for producing a DC signal having an amplitude which
indicates the frequency difference between an actual intermediate
frequency and a desired intermediate frequency;
(b) sample and hold means for sampling said DC signal to produce a digital
signal in response to said DC signal, said DC signal being sampled during
an RF pulse transmission and during a receive time; and
(c) microcomputing means for producing a control signal that modifies a
frequency of the variable frequency local oscillator, said control signal
being produced using said digital signal.
7. The apparatus of claim 6, wherein said sample and hold means comprises
an analog to digital converter.
8. The apparatus of claim 6, wherein said sample and hold means comprises a
comparator means for comparing said amplitude of said DC signal to a
plurality of reference voltages.
9. The apparatus of claim 6, further comprising a memory that stores a
temperature dependent value used by said microcomputing means to produce
said control signal.
10. The apparatus of claim 9, wherein said sample and hold means comprises
a comparator means for comparing said amplitude of said DC signal to a
plurality of reference voltages.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
In general, the present invention applies to radar systems. More
specifically it relates to the automatic frequency control (AFC) of a
variable frequency oscillator within a radar system.
2. Description of the Related Art
A radar system transmits and receives radio frequency (RF) energy. This
high frequency energy is difficult to filter and amplify. To facilitate
filtering and amplification, the RF echoes from targets are converted to
an intermediate frequency (IF) at which filtering and amplification can be
more readily accomplished. Conversion is accomplished by mixing the RF
signal with a signal obtained from a variable frequency oscillator.
The IF filters and amplifiers have a limited bandwidth. If the oscillator's
frequency drifts too much, the IF signal begins to drift outside of the
bandwidth of the IF filters and amplifiers, and thereby degrades the
radar's performance.
In the past, AFC was used to minimize the frequency drift of the
oscillator. This was accomplished by sampling the frequency of the IF
signal to verify that it was in middle of the IF bandwidth. If the IF
signal began to drift outside of this bandwidth, the frequency of the
oscillator was modified to make a correction. In the past, the IF signal
was sampled during the RF transmission of the radar's magnetron.
When the magnetron fires, a large amount of energy is transmitted from the
antenna into the atmosphere. Ideally all this energy is passed into the
atmosphere, however, some energy leaks into the receiver. The leakage
energy is mixed with a signal from the oscillator to produce an IF signal
that is sampled to adjust the oscillator's frequency. If too much energy
leaks into the receiver, there is a temporary shift in the frequency of
the oscillator. This shift lasts for the duration of the RF transmission,
and results in the oscillator operating at one frequency during RF
transmission, and at another frequency during the radar's receive time.
Since the IF frequency sample is taken during RF transmission, it includes
the temporary shift in the oscillator's frequency. By adjusting the
oscillator's frequency based on this inaccurate intermediate frequency
sample, the performance of the radar is degraded.
In the past, this problem was addressed by providing as much isolation as
possible between the magnetron and oscillator. Unfortunately, this
isolation is expensive, takes up a great deal of space, and has a
performance that degrades as temperature decreases.
SUMMARY OF THE INVENTION
The present invention automatically controls the frequency of a variable
frequency oscillator by obtaining a first error voltage that indicates the
frequency difference between a desired intermediate frequency and an
actual intermediate frequency. The first error voltage is obtained by
sampling the intermediate frequency during the transmission of a RF pulse.
Based on the value of this error voltage, a control signal to a variable
frequency oscillator is modified so that the actual intermediate frequency
approaches the desired intermediate frequency. During the radar's receive
time, a second error voltage is sampled where the error voltage indicates
the frequency difference between a desired intermediate frequency and a
second actual intermediate frequency. Based on this second error voltage,
the control signal to the variable frequency oscillator is modified again
so that the second actual intermediate frequency approaches the desired
intermediate frequency.
The present invention solves the aforementioned problems by making an
initial adjustment to the oscillator's frequency using a sample of the
intermediate frequency that is taken during RF pulse transmission. The
invention then uses reflections or echoes from nearby targets to obtain IF
samples that are not corrupted by the magnetron induced oscillator
frequency shift. Since this sample is taken at a time when the magnetron
is inactive, the additional isolation that was used in the past is no
longer necessary. Minimizing the amount of isolation reduces costs and
saves space while maintaining the radar's performance.
DESCRIPTION OF THE DRAWINGS
The FIGURE is a block diagram of a radar system and illustrates the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The Figure illustrates the present invention. The RF energy from magnetron
10 is fed to circulator 12. Circulator 12 has ports 14, 16 and 18. Energy
that enters port 14 is transferred to port 16, and energy that enters port
16 is transferred to port 18. The magnetron's RF energy enters port 14 and
exits port 16. The RF energy from port 16 is transferred to antenna 20 and
then transmitted into the atmosphere. Antenna 20 receives RF signals that
are reflected by targets. The received signals are fed into circulator 12
through port 16. The signals from port 16 exit port 18 into pre-amplifier
22. Pre-amplifier 22 performs an initial amplification of the RF signals.
The RF signals from pre-amplifier 22 are fed to mixer 24 where they are
mixed with a signal from variable frequency oscillator 26. The output of
mixer 24 is an IF signal having a frequency equal to the difference
between the RF signal's frequency and the oscillator's frequency. The
output of mixer 24 is fed to IF amplifiers and filters 27. At this point,
the IF signals are amplified and band pass filtered to remove noise. The
output of IF amplifiers and filters 27 is passed to detector and
discriminator 28. The detector provides a video signal that is used by the
rest of the radar system. The discriminator produces a DC error voltage
that is dependent on the frequency difference between a desired
intermediate frequency and the intermediate frequency received from IF
amplifiers and filters 27. The desired intermediate frequency can be any
frequency that is convenient for use with the components contained in IF
amplifiers and filters 27, but it is preferable to use an intermediate
frequency of 60 MHz. The DC signal produced by the discriminator, can
range from negative to positive voltages, but is preferable to have the
signal vary between 0 and 12 volts. In the preferred embodiment, a DC
amplitude of 6 volts results when the actual intermediate frequency is
equal to the desired intermediate frequency. If the actual intermediate
frequency is below the desired frequency, the DC voltage is between 0 and
6 volts, and if the actual intermediate frequency is above the desired
frequency, the DC signal is between 6 volts and 12 volts. The output of
detector and discriminator 28 is received by sample and hold circuit 30.
Sample and hold circuit 30 latches the output of detector and
discriminator 28 so that the information can be made available to
microcomputing device 32. Sample and hold circuit 30 converts the DC
signal to a digital signal. This conversion takes place using a plurality
of comparators that receive the DC signal on one input and a reference
voltage on another input. It is also possible to use an analog to digital
converter (A/D) to perform this function. Since the conversion time in
this application is relatively fast, a flash converter A/D should be used.
Since a flash converter A/D is expensive, it is preferable to use the
comparators and reference voltages. Microcomputing device 32 uses the
digital signal received from sample and hold circuit 30 to produce a
control signal which modifies the frequency of oscillator 26.
Microcomputing device 32 may be a microcomputer, a microcontrollor, or a
microprocessor; it is preferable to use a microprocessor. Microcomputing
device 32 also has access to memory 34. Memory 34 stores values that are
used by microcomputing device 32 in producing the control signal for
oscillator 26. Memory 34 can be a RAM, a ROM, an EEPROM, a UVPROM or any
other type of nonvolatile memory. It is preferable to use a ROM. If
oscillator 26 is equipped with a digital input, the control signal
produced by microcomputing device 32 can be fed directly to the frequency
control input of the oscillator. If oscillator 26 does not have a digital
input, the control signal is fed to digital to analog converter 36.
Digital to analog converter converts the control signal into an analog
control signal which is connected to the frequency control input of
oscillator 26.
When magnetron 10 produces a pulse of RF energy, the energy is fed into
port 14 of circulator 12. The RF energy then passes through port 16 and
out to antenna 20. Theoretically, all of the energy from port 14 passes
through port 16 and none is transmitted to port 18. In actual practice,
however, some of the energy from port 14 is passed to port 18. This
leakage RF energy or signal passes through pre-amplifier 22 and into mixer
24 where the RF signal is mixed with a signal from oscillator 26 to
produce an IF signal. After passing through IF amplifiers and filters 27,
the frequency difference between this IF signal and the desired
intermediate frequency is measured by the discriminator, and a DC output
signal is produced. This signal is then sampled and converted to a digital
value by sample and hold circuit 30. Microcomputing device 32 uses the
digital value to produce a control signal that modifies the frequency of
oscillator 26 in an attempt to make the actual intermediate frequency
equal to the desired intermediate frequency.
The control signal that results from the intermediate frequency sample
taken while magnetron 10 is active, is used to provide an initial
adjustment to the frequency of oscillator 26. This is only an initial
adjustment because this frequency sample includes an error which resulted
from a magnetron induced frequency shift in oscillator 26.
In a preferred embodiment, a modification is made to the initial adjustment
of oscillator 26 using the ambient temperature of the radar receiver.
Since the isolation between magnetron 10 and oscillator 26 is dependent on
temperature, the magnetron induced frequency shift of oscillator 26 is
also dependent on temperature. By experimentally determining the amount of
shift produced at different temperatures, a collection of frequency
correction values is obtained. Memory 34 is used to store these
temperature dependent values. By measuring the ambient temperature with
temperature measuring device 31 and then retrieving the proper value from
memory 34, microcomputing device 32 compensates for the frequency shift
introduced into oscillator 26 by magnetron 10. The number of values stored
in memory unit 34 can be any convenient number, but it is preferable to
provide values that cover the entire operating range of the radar in
5.degree. C. increments.
During the receive time of the radar, another adjustment is made to
oscillator 26. During the receive time of the radar, magnetron 10 is
inactive and does not induce a frequency shift in oscillator 26. The
intermediate frequency that is sampled to make this adjustment is obtained
using echoes which are received from targets within the radar's range. For
a radar that has a transmit power of 3000 watts, it is preferable to use
targets that are within 40 nautical miles of the radar. An intermediate
frequency sample taken during the radar's receive time permits a more
accurate determination of the difference between the actual intermediate
frequency and the desired intermediate frequency. Using this more accurate
measurement, microcomputing device 32 produces a control signal which
further adjusts the frequency of oscillator 26.
The IF signal sampled during the radar's receive time may contain a doppler
frequency shift induced by the motion of a target. In applications where
target speed is relatively low, for example 100 miles/hr, the inaccuracy
in the intermediate frequency measurement is insignificant with respect to
the bandwidth of the IF amplifiers and filters.
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